High-integrity, high-performance aerospace structures are often fabricated by overlaying sheets of metallic or non-metallic material and fastening the sheets to one another with fasteners, such as rivets or bolts. A series of connected sheets may be fastened together to form complex structures such as aircraft fuselages and fuel tanks.
When the sheets are fastened together, the surfaces that are placed in intimate contact with one another and become, for all intents and purposes, invisible without any discernable boundary, are known as “faying surfaces”. The respective faying surfaces must intimately mate with one another in order to provide a strong physical connection or bond between the sheets of material. The conformance of the faying surfaces is even more important if the resultant fabricated structure is to contain volumes of liquid or gas. For instance, the faying surfaces of an aircraft fuselage must prevent the escape of air from a pressurized cabin, and the faying surfaces of an aircraft fuel tank must prevent the leakage of fuel.
Sealant materials are often applied to faying surfaces and to fasteners disposed through the faying surfaces to provide improved sealing and impermeability to liquids and gases contained within the fabricated and assembled structures. Over time, sealants and methods of applying the sealants have evolved.
Liquid polysulfide resins are the most used faying-surface sealant materials because of their favorable chemical and physical properties, their ability to be pigmented, and their acceptance as an effective and efficient sealant system for use in the aircraft industry. However, since these sealant materials are applied in a wet viscous state, the coated objects being difficult to handle after having the liquid, viscous polymer resins applied to them. Further, the polysulfide sealants tend to degrade once in contact with high sulfur fuels.
Several alternatives to wet, liquid polysulfide sealant materials have been proposed over the years. Many of these alternatives use dry application processes and avoid the need for complicated wet applications. Nitrile-phenolic-based thin film adhesives provide for improved fuel tank sealing performance over the conventional wet, polysulfide sealing method. Also, sealants including fluoroelastomers, fluorosilicones, polyesters, polythioethers, polyurethanes, and polyureas have been developed. In addition, many technological advances in corrosion-inhibiting pigments, greatly reduced time and temperature curing parameters, elimination of fastener re-torquing requirements, and reduced environmental effects have been demonstrated with the new sealant formulations.
In the highly technical world of aerospace, there is an ever-present need for the significant and accurate prediction, quantification, and qualification of the various characteristics of materials such as sealants, in order to adequately compare their physical, chemical, and mechanical properties. Although a variety of sealant materials are now available, there is no uniform basis available to compare the physical and mechanical properties of one sealant material versus another. For instance, standard measurements of viscosity do not adequately describe the stress-relaxation of a cured polymer compressed between two faying surfaces. Further, standard measurements of elasticity do not adequately represent the characteristics of sealant materials under conditions of repeated compression and relaxation.
It is, therefore, desired to provide a method of testing sealant materials under standardized conditions that results in measurements and quantifications of the stress-relaxation characteristics of these materials for aerospace applications, such as between faying surfaces of structural components. It is further desired to provide a method of testing the stress-relaxation characteristics that enable the comparison of one material with another in terms relevant to one skilled in the art of faying-surface coatings or, more particularly, the use of faying-surface coatings in aerospace applications.